Fig.1 What a sweet cuppa?
“Why is the tea sweet?”
“Cause of the sugar”.
“How do you know there’s sugar?”
“Surely the taste is there.”.
Sugar is invisible in tea yet the sweet taste is sufficient proof for its presence. When a solid substance goes into hiding inside a liquid, we call that process dissolving. This is not a change of state due to a change in temperature or pressure. Dissolving require at least two substances.
Solvent + Solute dissolve to give a Solution.
A substance that can dissolve is a soluble substance. One may be highly soluble, slightly soluble or insoluble
Results of an experiment to compare the solubility of a few solids
Ex: 1
Method:
Take a bottle for each substance half filled with rain water and label them. Mark the level carefully and add a little at a time until a little remains visible. After noting the colour and the clearness or visibility through the mixture note the level and leave it undisturbed for a few hours. Record the changes. (The substances mentioned are examples and you are free to change)
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Salt
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Lime
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Soap coloured
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Condys
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Step 1. Sedimentatiuon
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Observations
On stirring
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No change.
Clear
No particles remain.
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White
Opaque
Some remain.
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Foamy
Clear but coloured
No particles.
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Clear but Pink.
No particles remain.
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After sometime.
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No sediment
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White sediment
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No sediment
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No sediment
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Step 2. Filtering
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Salt
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Lime
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Soap
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Condys
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Filtrate
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Clear and colourless filtrate
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Clear and colourless filtrate
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Clear but lightly coloured.
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Pink but clear.
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Deposit on filter paper
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No deposit.
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White deposit
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No deposit
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No deposit
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Step 3. Evaporating the filtrate.
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Salt Lime Soap Condys
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Observation
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white patch
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white residue
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residue
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dark residue
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Conclusion
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Soluble
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Slightly soluble
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soluble
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soluble
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Characteristic properties of a solution formed by dissolving a solid in a liquid medium.
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1. The solid dissolved is invisible.
2. If the solute is completely dissolved the solution has to be clear. If not dissolved, a light ray passing through can be seen. (Tyndall effect)
3. The solute or the solvent can impart a colour but a true solution cannot be white.
4. A solution can pass through a filter paper without leaving a residue.
5. On evaporating the filtrate the solute would be left behind.
6. The solvent may be recovered by distillation.
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Is dissolving a Physical or a Chemical Reaction ?
Dissolving usually is a physical reaction. Here are a few instances of chemical dissolving:
1. Egg shells in vinegar. ……….CaCO3 +2 CH3COOH → Ca (CH3COO)2 + H2O + CO2
2. Quick lime in water. ………. CaO + H2O → Ca(OH)2
3. Zinc in Hydrochloric acid. Zn + 2HCl → ZnCl2 + H2
Solubility
This is the mass of the solute that can dissolve in 100g of water at a particular temperature.
Most substances have a high solubility at a high temperature.
The solubility of gases decrease with the temperature.
Saturated solution
The condition reached by dissolving the maximum amount of solute at a particular temperature is termed ‘saturation’. By heating a saturated solution it is usually possible to dissolve a little more. On cooling the excess has to precipitate. In some substances this may not happen. Then the solution is a ‘super saturated solution’. One such a substance is ‘sodium thio sulphate’ (hypo).
Crystalization
If you add a crystal of the same substance to such a super saturated solution, it would be amazing to see a cluster of crystals coming down in a jiffy.
Even if you allow a warm saturated solution to cool slowly Crystals will come down. You can try this with copper sulphate. If one of the tiny crystals attached to a string and kept in the saturated solution. The crystal will grow. The ideal substance for this is alum. Potassium aluminium sulphate.
Making crystals
The graph below shows the solubility of normal cane sugar as the temperature rises. If you use a clean bottle , a clean piece of yarn, sugar from the cupboard and pure water you will get brilliant crystals that you can eat. The colour can be chosen using some food colour.
Method
The graph above gives the quantity of sugar required for 100 ml of water. Add the warm water into a bottle or a glass. ( If the water is too hot the bottle will crack) Then add the sugar stirring all the time. When you find no more is dissolving decant the clear liquid in another bottle. If you leave any crystals at the bottom they will grow without forming new crystals. Now dip a piece of cotton yarn as shown and keep the liquid undisturbed. It is important to cool as slowly as possible in order to get good crystals.
Gases dissolving in liquids
If you keep some fresh cool water in a glass inside a warm room tiny bubbles will appear. This is not evaporation but the dissolved gases getting out due to heat. There is an easy test to find out weather a gas is soluble or not..
Method
Fill the gas you wish to test into a test tube and invert it in a cup of water. Care should be taken not to allow the gas to leak out.
Solubility of some gases at 20 C and atmospheric pressure.
Oxygen …. 0.045g/kg , Carbon dioxide……..1.7g/kg, Nitrogen…….0.02 Chlorine….7g/kg, Hydrogen…0.0016 (Courtsey..Engineeringtool box.com)
Solubility of gases decrease with temperature and increases with pressure.
Liquids dissolving in liquids …(Miscible liquids)
If you mix 20ml of oil with 20ml of water you will get 40ml a mixture. Within a few minutes the two liquids will separate. That means they are not soluble in each other. But if you mix 20ml of ethyl alcohol with 20ml of water you will not get 40 ml. There will be reduction of volume in dissolving a liquid in another liquid.
1.0 What kind of mixing would occur when the following are mixed?
1.1 Salt and water.
1.2 Garden soil and water.
1.3 Coconut oil and water.
1.4 Egg shells and lime juice
1.5 Carbon dioxide and water (20 Marks)
2.0 Three solid substances A,B and C were tested as follows:
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TEST
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A lime
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B
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C
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Mixing..
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with water gave a milky liquid.
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with water gave a clear transparent liquid.
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with alcohol gave a dark liquid.
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Filtering
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Clear filtrate.
Residue present
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Clear filtrate.
Residue absent
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Dark liquid
Residue absent
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Evaporating filtrate.
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White
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White powder.
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Dark deposit.
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Conclusion
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……………….
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……………….
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……………….
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(15 Marks)
3.0 Multiple choice
3.1 When a solid dissolves in water which is not likely to happen?
A- Sedimentation B – Increase of density C- Get coloured D- Colourless
3.2 Water taken at 20°C was warmed to 30°C degrees some bubbles appeared. Which can explain this phenomenon.
A- Evaporation. B- Decreasing solubility C- Increasing solubility D- Condensation.
3.3 When a crystal of the solute is added to a saturated solution what may happen?
A- Dissolve B – Crumble C - Grow D – Get super saturated.
3.4 Adding a sugar crystal to a sugar solution resulted in a large cluster of crystals
forming. Which is the correct description of the solution.
A- Dilute solution. B- Concentrated solution C- Saturated solution D- Super
saturated solution.
3.5 If an aqueous solution containing the maximum amounts of sodium chloride and potassium nitrate at 100°C was cooled to room temperature, what can you say about the residue formed?
A- A mixture of NaCl and KNO3, B- Mostly Nacl C- Mostly KNO3. D- Mixed crystals.
(25arks)
4.1
100g of water was saturated at 40°C with potassium nitrate . If the temperature of the
solution was raised to 50°C, how much more solute would be necessary to saturate it.
(Use solubility curves given)
4.2
50ml. of a saturated solution of potassium chloride at 90° C was cooled to 40°C.What mass of solute is likely to precipitate.
(20 Mars)
5.0 Find suitable names to label the diagrams.
5.1
5.2
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